In Castle Valette, you can summon an infinite number of "Dancing Sword" and "Dancing Crossbow" enemies, by setting the orb-pairs to be red. They are mean enemies, especially if your levels or equipment are lacking any bit... But the Crossbows drop Placidus Wood quite liberally!! This is probably the earliest you can get them, aside from the harder version of the same enemy in the Seraphic Gate's Castle Prevant floor.